The main determinant of the shelf life of many foodstuffs stored at ambient or chill room temperatures is microbial spoilage. The rate at which food deteriorates through the action of microbes is governed by a great variety of factors, both intrinsic and extrinsic. Generally, however, the single greatest factor that determines the rate of microbial growth (and hence the shelf life of the foodstuff) is the total number of microbes present on the foodstuff when it goes into store. In order to maximise shelf life, it is important to ensure that the foodstuff is exposed to as low numbers as possible of microbes.
Good practice in the food industry therefore demands scrupulous attention to cleanliness of raw materials, handling equipment and work surfaces, packaging, and, perhaps most importantly, personnel. Hygiene is also important from a public health point of view: many foodstuffs provide excellent substrates for the proliferation of pathogenic micro-organisms, which can lead to food poisoning as a direct result of transmission of the microbes from the foodstuff to the human consumer, and/or through the action of toxins produced by the microbes on the foodstuff as a by-product of their growth.
Meat is a prime example of a foodstuff in which microbial growth is of tremendous importance, because it is an excellent substrate for and transmitter of food poisoning organisms. Although the animal muscle that is the forerunner of a cut of meat is itself essentially sterile in a healthy animal, post-slaughter microbial contamination is inevitable. Typically, the process of breaking down a carcass of a meat animal to portions suitable for consumption or further processing involves many separate operations, many of which are still carried out manually. This is true for whole-meat cuts, such as steaks, chops, roasting joints, etc, but is enhanced if the meat is to be further processed, where frequently comminution will occur which by its very nature increases the surface area of the meat that can be in contact with microbe-harbouring machinery and work surfaces. Several economically important groups of meat products include during the course of their manufacture microbicidal or microbistatic factors: among the most noticeable being cooking, mixing with curing salts, and reduction of available water (for example through drying); despite this, such foodstuffs are all too frequently implicated in food poisoning outbreaks, when recontamination of the foodstuff after processing is often blamed. Cooking of any meat cut or product will reduce microbial numbers, the effect being largely determined by the combination of the temperature reached during the cooking process, and the overall time/temperature history, but the self-evident changes that heat treatment of this sort bring about in the meat itself limit the potential for using heat as a method of reducing microbial numbers. Nevertheless, it has received wide spread application, in particular for the development of food products with a very long shelf life. Usually such products have a very high fluid content, facilitating dosage of the combined foodstuff in aseptic packages.
It has long been known that irradiating foodstuffs with ultraviolet rays, especially with wavelength of 235-280 nm, can be used for reducing the microbial load on foodstuffs. This effect has achieved some commercial success in the large scale storage of certain foodstuffs, especially cheeses and other dairy-based products, where comparatively large quantities of the foodstuff are stored chilled under constant ultraviolet irradiation. This can be particularly beneficial for foodstuffs where mould and other fungi are the primary causes of spoilage. In these circumstances, the conditions of ultraviolet radiation have been selected primarily to hinder the growth of microorganisms, rather than eliminate them.
Ultraviolet light is only effective on the surfaces of foodstuffs, since it has very little penetrative power. This inability of ultraviolet radiation to penetrate far into any material is possibly the root cause of the comparatively modest success of ultraviolet treatment generally in the food industry. Most non-liquid foodstuffs have convoluted surfaces with multiple invaginations which harbour microbes that cannot be reached by UV irradiation. The vast majority of packaging materials currently in use in the food industry are generally effective barriers to UV penetration; packaging materials typically used in the meat industry, for example, may only allow about 5% of the UV irradiation falling upon their surface through to the packaged meat. This general opacity to ultraviolet light of most packaging materials in the food industry limits the potential of processes which seek to utilise the microbial reductive powers of ultraviolet irradiation on packaged foodstuffs. UV-transmissible food-grade packaging materials do exist, but their total use in the commercial market place is not high.
Tanaka (U.S. Pat. No. 4,983,411) teaches vacuum packing raw meat in a UV transmissible film, exposing the vacuum packed meat in its package to ultraviolet irradiation, and then shrink-wrapping the package around the raw meat in a high temperature atmosphere. Tanaka therefore proposes packaging, followed by sterilization. The present application addresses the sterilisation of a foodstuff with subsequent packaging, and the packaging of foodstuffs under conditions which maintain the microbially reduced status of the foodstuff after irradiation in an efficient, simple, and cost-effective way. Tanaka uses broad-spectrum UV-radiation, including long-wavelength radiation (300 nm and greater). Such radiation penetrates packaging film relatively easily but is not very effective at impairing or killing microorganisms. Any such activity is likely to be non-specific, e.g. relying largely on a heating effect. Such irradiation is likely to have deleterious effects on meat quality, e.g. associated with photochemical oxidation of lipids and/or pigments.
Vacuum packaging of meat can often induce the meat to "weep"; that is, a fluid is drawn from within the meat pieces by the vacuum and collects between the meat surface and the inside of the packaging. Weep extruded onto the meat surface can inhibit the transmission of UV-radiation, thereby reducing the effectiveness of UV-irradiation of vacuum packaged meat as a system for reducing microbial numbers.
Another potential drawback limiting the widespread application of ultraviolet irradiation for reducing microbial loads on meat and meat products, and other foods, is the potentially deleterious effect on colour and lipid oxidation. This latter aspect is likely to be enhanced by the smearing of adipose tissue on the inner surface of UV-transmissible bags that are heat shrunk following evacuation of the air.
A further barrier to the commercial success of UV irradiation processes for reducing microbial numbers on foodstuffs has been the expense associated with the dedicated equipment required.